Room temperature vulcanizable (RTV) silicone rubber compositions combining the properties of high application rate (i.e., easy to extrude and pump) and good thixotropy (i.e., non-sagging in vertical joints) have a significant advantage in the marketplace. It is therefore desirable to provide a one-component RTV silicone rubber composition having both high application rates and good thixotropic behavior.
RTV silicone sealants frequently achieve thixotropic character by the presence of high levels of fumed silica reinforcing filler. However, the level of fumed silica required for thixotropy (i.e., &gt;12 wt. %) frequently causes drastically reduced application rates. Also, RTV silicone sealants, utilizing lower levels (i.e., &lt;10 wt. %) of fumed silica as the principle thixotrope, frequently demonstrate adequate thixotropic behavior only for a short period of time immediately after compounding. However, upon post-production storage and subsequent re-shearing of the RTV (for example, as caused by high pressure pumping during re-packaging) the thixotropy of the RTV can be significantly reduced to the extent that the RTV will flow out of a vertical joint. Reference is made, for example, to U.S. Pat. Nos. 4,483,973 and 4,528,353 (Lucas et al.) and to U.S. Pat. No. 4,472,551 (White et al.). In the RTV compositions disclosed in the Lucas and White patents, no thixotropic additives are used with the exception of fumed silica. The Lucas and White compositions are typically non-sagging as initially made but lose their thixotropic properties upon experiencing post-production shear resulting in flow out of vertical joints.
It is common practice, in the RTV industry, to utilize organic glycols, such as polypropylene glycol and polyalkylene glycol as thixotropes. However, there is no known prior art that describes the use of low viscosity, silanol terminated, linear dimethyl silicone oligomers specifically as RTV thixotropy enhancing agents.